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What is a rubber engine?
I hear it a lot in engine design , I think that belongs to some phase about we can change the engine parameters according to requirements but I am not sure about it.

A rubber engine is not to be confused with a rubber motor (a rubber band which is twisted in order to store energy which can be released when the rubber band untwists. This type of engine is good for short energy bursts in small model airplanes).
A rubber engine is an engine deck (tables of engine data) which can be scaled according to your needs. In a way, you stretch one parameter of the data like a rubber band, and all other parameters change accordingly. In most cases, you adjust thrust to your needs and the scaled engine deck will produce mass, size and fuel consumption for the most likely actual engine with the given thrust. This is very helpful in the initial stages of engine design when you search for the smallest possible airframe to fulfil given mission requirements.

So, just to make sure I understand correctly, a "rubber engine" is a particular type of data table?

@TannerSwett - that's correct.   It's not a "physical device".

I intended to make this a comment on Peter Kämpf's answer, but don't have enough reputation.
In my engineering education, a "rubber engine" referred to an engine with variable parameters. A "fixed engine" referred to a specific engine with well-defined parameters, for example "Lycoming O-235-F1." In general, you use a rubber engine in aircraft design when you know generally what type of aircraft you're working on, e.g. a single engine piston-propeller or a multi-engine turboprop, and you just need values that are "good enough" without restricting your design to a specific model. Once you have an idea of what the aircraft's parameters are going to be, you can select a specific engine model and modify the rest of the design to work with the newly-restricted engine.
It's not as restrictive of a definition as a type of data table; it can be as simple as having single numbers for power, fuel consumption, weight, etc.

Basically a "rubber engine" is a concept.  It's the idea that your plane has an engine, but it doesn't have locked-in specs.

"Single numbers" can be viewed as a (small) table, so Peter's answer still applies.

@CarlWitthoft Yes, Peter's answer does still apply, I only wanted to point out that one doesn't need to generate/have "tables of engine data" (i.e. more than one set of numbers) to employ the rubber engine concept. Additionally, single parameters can be modified without having "all other parameters change accordingly." While scaling the other parameters might be a good idea, ultimately it's up to the designer to keep the engine specifications reasonable. Peter's answer is correct, but in my opinion might convey too strict of a definition of what the rubber engine concept is intended to be.


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Flight Plan - Rights of use and distribution to third parties
Currently we have in mind a meteorological project, specially designed for pilots and aviation professionals, for which, the only thing that we currently lack is to have the flight plans. It seems to be (as Eurocontrol tells us) that flight plan information is only distributed to airlines and air traffic control companies, not to third parties. Therefore, the only thing that remains is for the pilot to provide us with the flight plan.
What use rights does a flight plan have? Is there a problem in which the pilot shares this information with third parties?

Welcome to aviation.SE! IMO, usage rights are a minor issue compared to what you can offer and how it would integrate into existing flight planning tools. In other words, market research and product development. As a Q&A site (see the [tour]) we don't encourage discussions, so I think your idea about talking to a pilot (preferably to a lot of pilots) is a good one. But if you have specific, objective questions that would fit the format of this site then please do ask them.

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What‘s the thrust of two stacked propellers?
Let‘s say that you have two identical propellers and each is powered by their own motor and together they produce a thrust of 1 kN. How does the thrust change if instead of operating them side on side I stack them one over/behind the other? I‘m sure they wouldn't still produce 1 kN but how much would it approximately be?
Furthermore, what if I added another propeller of this type and so on?

related: https://en.wikipedia.org/wiki/Contra-rotating_propellers . For the same rotation direction, efficiency is probably horrible (curious on input here).

How would you stack the propellers if they each still had their own motor?

Thanks, but this article doesn‘t answer my question since contra-rotating propellers are an alternative to one bigger propeller. However, the comparison I‘m looking for is a pair of contra-rotating propellers vs two of those same propeller side by side. You can basically stack as many propellers as you want, just gotta fit an electric motor between them

@Pilothead I've seen a bunch of larger drones with three or four heads consisting of two motors back to back with a propeller at top and bottom.  I'm pretty sure this is what the OP is asking about.  Further, in those cases, all the propellers are generally identical (with regular contra props like on the Bear bomber, the rear one has a different pitch setting and often a different blade profile to compensate for running in a rotating slipstream)

This configuration, 2 props driven by 2 engines, was used on the Fairey Gannet, and on the inboard props of the Bristol Brabazon. So it's an interesting question.

There's a really interesting paper by Manikandan Ramasamy (Hover Performance Measurements Toward Understanding
Aerodynamic Interference in Coaxial, Tandem, and Tilt Rotors, Journal of the American Helicopter Society, Vol. 60, 2015) that talks about this question, which is of great interest for multirotor UAS (which, for rotorcraft, is a substantial number of aircraft): how does stacking two rotor change their efficiency and thrust output?
His test apparatus looked like this: two rotors located exactly one on top of the other, and his apparatus allowed him to measure both torque and thrust produced by each one independently.  He could also vary the distance between the two rotors.  For reference, the rotors are ~4.3 ft in diameter.  Also, just to explain the graphs I'll throw up from his paper, he used blades both with and without twist.

The reason why he has torque sensors is going to come into play with the results.  For a coaxial rotor helicopter (not necessarily for a coaxial prop airplane -- I am not sure which you care about), it is desirable to have a coaxial rotor setup wherein each rotor produces the same amount of torque.  Multicopter UAS then disturb that torque balance to control themselves in yaw.  For the tests Ramasamy performed, they were all done when the rotors were torque balanced, so each rotor produced a torque equal and opposite to the other rotor.
His results are pretty striking, as shown below.  This is a plot of how "interference factor" for each rotor changed with respect to how close the rotors were to each other.  "z/D" refers to the vertical separation of the rotors, z, as a fraction of the rotor diameter.  The interference factor is a measure of the extra power required to turn the upper and lower rotors, ($\kappa_u$ and $\kappa_l$, respectively) as compared to an isolated rotor.  An interference factor of 1 means the rotor is equivalent to an isolated rotor.  Greater than 1 means that it is less efficient than an isolated rotor and vice versa.

As you see, as the rotors get further apart, the upper rotor acts as if it is all by itself ($\kappa_u=1$).  However, the lower rotor is always more inefficient than an isolated rotor due to downwash from the upper rotor.  That said, as the two rotors get closer, the effects get a little more wild.
As the lower rotor approaches the upper rotor, it helps pull air through the upper rotor, so you might think that the angle of the air hitting the blades is getting steeper (also called a higher inflow angle).   This also leads to a lower angle of attack on the blades, leading to the following effects for the upper rotor: decreased thrust production (reliant on high angle of attack to produce more lift) and increases torque input (reliant on inflow angle).  Consequently, $\kappa_u$ increases.
However, since the upper rotor is producing less thrust, the downwash on the lower rotor decreases...and, as you might imagine, it sees the opposite effect that the upper rotor sees: higher thrust production, lower torque input, and $\kappa_l$ actually decreases.  Due to this, you now need to either slow down the upper rotor or speed up the lower rotor in order to achieve a torque balance.
Hence, the first answer to your question is...it depends, but it looks like you actually can get a more thrust out of the coaxial configuration (as opposed to two isolated rotors) if you can space the rotors out well.  This is due, at least in part, to some "swirl recovery" performed by the lower, counter-rotating rotor.  In other words, since a rotor not only pushes air down but also forces it to spin (like a tornado), the lower, counter-rotating rotor sees faster moving air and can take advantage of this to operate at a lower angle of attack: producing the same amount of thrust with lower drag penalties.  However, the effectiveness of this, as said, varies with the separation of the two rotors.
And, again, the larger issue, I think (from a performance perspective) in coaxial setups is not thrust production but power requirements.  One of the two rotors (if not both) is always going to be less efficient than an isolated rotor producing the same thrust.  This is shown more clearly below in a comparison of Figure of Merit (Fig. of merit = (Idealized power requirement)/(Actual power requirement), so a measure of rotor efficiency) versus coefficient of thrust (analogous to the amount of thrust produced by the rotor) for the rotors with untwisted blades.

In short, while you can get more thrust out of a pair of coaxial, counter-rotating rotors, there is no free lunch.  And the lunch can get pretty expensive, at least from a performance point of view (not to mention the extra structure, etc. required for larger axial separation distances).

Wow thanks, what a great answer ! This rely lets me look forward to my own experiments on this subject :)

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How are the takeoff V speeds calculated?
What is the actual formula for calculating the takeoff V speeds of an aircraft with any configuration? Specifically, I mean V1, Vr and V2
I know there are many factors, but what are they specifically and what operations are used on them in an overall formula?

Im sorry. I was acting as if everyone knew what I meant :/. I meant to be speaking of the most commonly known -- that is "V1" "VR" and "V2". I have edited my question accordingly.

See the following answer: How is minimum runway length related to V1?.  They don't actually use a formula to determine them, they determine them through actual flight testing (with some limits specified in the certification rules).  While that answer doesn't address V2, it goes in to detail on the others and contains links to the regulations.

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What is the purpose of the plush-like seat covers in an airliner cockpit?
What is the purpose of the plush-like seat covers that I have seen on almost all the airliners I looked at? See for example the following picture of a Bombardier CRJ flight deck:

(wikimedia.org)

Check out this related question
And this forum topic.
They help prevent wear and tear on the seat fabric underneath and are fairly easy to replace. In a lot of GA planes they are installed to hide old tired seats underneath. But mostly they are just really really comfortable. They breathe in warm weather and help insulate in cold. All around a good fabric choice for the application (see related question for notes on fire prevention).

It's not just the airlines.  I have them in my Cessna Cardinal, similar to these https://www.shearcomfort.com/Airplane-Seat-Covers.asp?utm_source=google_shopping&utm_medium=cpc&gclid=EAIaIQobChMI3s2Hpfqt3AIVUC-BCh0dxQD3EAQYASABEgKKOPD_BwE  Mine are 20 years old, I've long ago forgotten what brand, but cool in summer and warm in winter. I haven't flown in a few years, I'd forgotten how comfortable they are now that I am flying again.

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How to improve manual flight skill?
I've got a question quite specific, and directed to expert pilots:
How to improve a manual skill during solo flights?
I understand that take an instructor with me would probably be the best way, but I need to save money for the incoming CPL course.
So any suggestions?

practice makes perfect

This is very broad. What skills are you wanting to improve, and why do you think you won't improve by practice?

To expand a bit, set yourself a higher requirement. E.g. Don't shoot for +-100 feet, aim for +-10 feet.

This seems like a very broad question that will get a lot of opinions. But as a general comment, fly with purpose: plan what you want to do, what performance is acceptable etc. and debrief in detail afterwards, preferably using some kind of GPS tracking data. If you're instrument-rated, flying by hand under IFR is demanding and a great way to push yourself to fly with precision, especially in IMC or under the hood.

Take an aerobatics course.

@RonBeyer, the question states that money is a consideration, an aerobatics course is not a way to save money.

@GdD Sure it is, a couple hours with an instructor in a 150 Aerobat is worth quite a few more developing bad habits solo without iinstruction.

I totally agree that instruction of any kind is a good thing @RonBeyer, I'm simply pointing out that the question is how to improve without it.

Don't use the autopilot, use the trim, and look outside!  Fixating on the instruments will not make you better.  Know the sight picture out the windshield and what wing looks like in relation to the horizon.  Set 25' altitude brackets and 2° heading brackets and hold yourself to them.  It'll seem impossible but you'll get better.  Set those brackets with the intent to hold them by looking outside, not chasing needles in the cockpit.
Fly using the actual horizon as your reference, which is safer anyway, and make minor adjustments based on a periodic instrument scan that should become less and less frequent as you get better (think 10-15sec between scans to start).  Hold heading based on distant objects, not the compass or DG.
Finally, use the trim.  If it takes more than two fingers to adjust pitch, you are not trimmed.  If you fly a plane with a trim wheel between the seats get used to resting your hand on it.  Every time you change the power setting, retrim.  In an updraft?  Retrim.  Down draft?  Retrim. It'll become second nature in no time.

For this, there is just no substitute for practice.  The phrase "to improve manual flight skill is a pretty broad request for a large variety of basic airmanship skills so the only way I could help answer this with a few paragraphs is to concentrate on one specific skill.
Remember that a pilots license or rating is just a journeyman's license to practice basic airmanship; flying an aircraft is an art which takes a lifetime to master.  There are pilots with over 20,000 hours total time which learn something new on every flight.
I might recommend the following general tips for you:
Practice! Practice! Practice!  Work on specific maneuvers and skills which you are struggling with and perfect them.
Consult the FAA's books such as the Airplane Flying Handbook or the Pilot's Handbook of Aeronautical Knowledge for information on good, basic airmanship.  They're very well written and contain a wealth of knowledge on these subjects.  Best of all, they're free online!
No man is an island; talk with other pilots and flight instructors about these subjects and learn from them.  Build up this ecosystem of knowledge and always keep expanding this base.
Keep current!  Fly on a regular basis and practice different skills e.g. Maneuvers, navigation, pattern work, etc.  and don't forget about flight reviews.  Do a BFR every or an IPC year with an instructor, whether your current or not.  It will help weed out bad habits that slowly grow into your flying routine.
Set personal minimums and periodically evaluate these for safe operations.
Use simulators to perfect techniques once an instructor has demonstrated them to you in a real aircraft.  FSX, X-Plane, Prepar3d, etc may be games but they use extremely accurate flight models and can be used to perfect a lot of basic airmanship.  Note:  fly the techniques your instructor taught you in the sim; do not self teach.  You can get a lot of bad habits ingrained if you do not.


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During the holding procedure is it required that the pax must have the seat belt on?
Imagine to have to hold at a busy airport before landing for a quite long time (let's say 20 plus minutes), does it requires that the pax must have the seat belt on during the whole holding procedure? Is it required by the Authority or just by the company?

Commercial airliner? Need to do what the Captain shows via the seatbelt sign.
It's a good idea to have it on all the time anyway, never know when unexpected turbulence might hit, such as wake turbulence from another plane, or your own even.

There are no regulations which require passengers to be seated when the plane is flying a holding pattern, but there are a few practical advantages to keep the seat belt sign on during this phase:

The plane is flying at a lower altitude, where it is more likely to encounter turbulence and other unstable weather conditions. Up in FL330 it is usually clear skies all along.
The plane may need to get down quickly after an approach clearance is issued (after all, it is busy airspace). From that point onward the pilots may deploy the spoilers to lose altitude (which shakes up the plane a little), turn towards the approach course and without you noticing, landing gear is down and locked already. At the same time, we know it takes quite a few minutes after the seat belt sign has been turned on until everyone is seated (passengers won't just rush back to their seat after hearing the Ding Dong tune). If the seat belt sign is not illuminated in this period, there may not be enough time for the cabin crew to prepare the cabin for landing.
It reminds the passengers that if they wish to get out of their seat for whatever reason, they should make it quick.

If the flight deck tells the passengers it is going to be a delay, the cabin crew is usually a bit relaxed at enforcing the seat belt sign. E.g. if someone is going to the bathroom, they won't block unless that person is hanging around standing. When the approach clearance is given, another announcement will be made, and the cabin crew will now make sure everyone is seated because landing will commence shortly.

"3. It reminds the passengers that if they wish to get out of their seat for whatever reason, they should make it quick." This is wrong. When the seatbelt is on, the passengers are not allowed to get out of their seat for any reason. There should be absolutely no exceptions given. Otherwise a great answer.

There is no requirement for seatbelts during holding.
However, holding times are really hard to predict. It is good to be prepared and to have the cabin ready for approach, so if an opportunity for a quick approach comes, it can be taken.

Seat belts are only required for ground movement, takeoff and landings.
For the US and presumably other countries too, the only requirement is given under FAR 91.107 which says,
"(3) Except as provided in this paragraph, each person on board a U.S.-registered civil aircraft (except a free balloon that incorporates a basket or gondola or an airship type certificated before November 2, 1987) must occupy an approved seat or berth with a safety belt and, if installed, shoulder harness, properly secured about him or her during movement on the surface, takeoff, and landing."

And here's why you should wear them all the time:  "Southwest passenger died after broken plane window nearly sucked her out" https://www.cnn.com/2018/04/18/us/southwest-emergency-landing/index.html


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Is Doppler navigation still in use for civilian aircraft?
Is it possible to find an airliner nowadays with Doppler radar used for navigation purposes?


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What should be the minimum time spent in cruise (for e.g. a B737)?
Having a short sector to fly, it would not be convenient to climb all the way up to the best cruise altitude, since the time in climb would burn more fuel than that necessary for a flight at lower altitudes.
Having said that, what would be the minimum time spent at the best cruise altitude that will make it reasonable to climb to that pressure altitude with a medium range twin jet transport airplane (B737 or similar)?

The operators I‘m familiar with elected to define a minimum cruise time of 5 minutes in their flight planning policies.

@Cpt Reynolds could you provide the type of airplane please? just as reference.

I believe it is uniform for all aircraft types of these operators - various Airbus and Boeing models of all sizes.

Also, generally, I believe for most types on short trips climbing to altitude even for only a minute then gliding down to destination is more efficient than a longer cruise at lower altitudes. Yes, you burn more fuel in climb, but less in descent, and with a shallower descent than climb that trade should be beneficial.

Well... I would use a different airplane model :)

In short-range flights: Ideally zero. Practically, a few minutes are sensible in order to transition between climb and descent. In longer-range flights there is initially no real difference between climb and cruise, because the aircraft climbs at or near its speed for best range until ATC or envelope limits forbid it to go any higher. Note that the Breguet range equation requires the aircraft to climb until the end of its flight.
Yes, a climb needs more fuel but will increase potential energy, too. This energy can be made good use of by flying at a lower thrust setting during descent. Now it is important at which speed the aircraft climbs: If its ground speed is not lower in a climb than in cruise, it will not lose anything by climbing.
Flying higher translates into a higher ground speed for a given indicated speed and an improved thermodynamic efficiency. Since the most efficient cruise speed is at a given lift coefficient and in most cases quite a bit lower than the maximum level speed at full throttle, flying fast at low altitude is very inefficient. Both, the best cruise speed and the maximum speed, converge with altitude until both fall together in the coffin corner.
If the aircraft flies at best range speed and full thrust, the excess power can be used for climbing. This short Gedankenexperiment makes it immediately attractive to climb - the aircraft covers the same ground while improving its ability to fly faster. In addition, applying full power will most likely improve engine efficiency a bit.
This will make climbing higher more attractive until you hit either of two limits:

The temperature stops decreasing with altitude, so any increase in altitude does not improve thermodynamic efficiency anymore, or
The flight lift coefficient rises above the value at optimum cruise, so the aircraft will cruise less efficiently when flying higher.

Both cases require a lot of thrust, and in most practical cases the engines will run out of oomph with altitude before one of the two limits is reached. Also, in most practical cases the aircraft will fly a little slower than its optimum range speed in order to improve its climb rate, so it will spend more time at more attractive altitudes.
The maximum altitude in short flights is determined by the rate of climb and rate of descent: The aircraft will climb until it reaches the altitude and distance to the destination at which flight at idle power will bring it all the way to the destination. In longer flights it is engine performance which limits the climb speed, but ideally the aircraft should continue to climb as it sheds mass by burning off fuel.


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Why in the Seneca 2 the Rated Horsepower are higher at 12000 feet than at sea level? (215Hp at 12000feet, 200Hp at sea level)
Reading the POH I found that the Rated Horsepower at 12000 Feet are higher compare to sea level. I have in the past studied the back pressure of the exhaust gasses but I cannot completely understand how it works.

Could someone explain me in detail how it properly works, please?

Back pressure affects power because the more pressure opposes the exhaust, the more exhaust gas is left in the cylinder when the exhaust valve closes.  This trapped exhaust gas, effectively inert for purposes of the engine cycle, limits the amount of intake charge that can be drawn from the manifold at a given manifold pressure.
The more intake charge you have, the more power you get (within limits), so the less unscavenged exhaust you have diluting the intake charge, the more power you can generate for a given manifold pressure.
This effect can be mitigated to some extent with valve overlap -- where the exhaust valve remains open for a short time after the intake valve opens; this allows the incoming intake charge to push some additional gas out through the exhaust valve -- providing there isn't too much "back pressure" on the exhaust side.

This depends on the engine. The Seneca II was equipped with  A dedicated turbo charged engine, which made use of turbocharging through its entire flight regime.  This was not a turbo normalized installation.

@CarloFelicione I removed the turbo reference entirely -- turbocharging only matters in terms of keeping manifold pressure up, and for a given manifold pressure, the lower the exhaust pressure, the better.

I’m not totally sure here, but a clue does come from the Systems Description in Section 7 of the Seneca II POH.
The induction manifold is equipped with a pressure relief valve to prevent overboosting by the turbos at low altitudes.  There is also a bypass orifice which sets to a maximum of 40” MAP, full throttle, at 12,000 feet.
Another factor is that the Seneca II POH recommends a maximum throttle setting of 39” MAP for a sea level takeoff and cautions not to exceed 40” MAP during this process.  This restriction is placed until clearing 12,000 feet pressure altitude.  Apparently throttle setting of a maximum of 40” MAP are allowed at this point all the way to critical altitude.
Which would probably explain this power discrepancy.


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Under what conditions can a pilot log simulated instrument time?
Can a pilot log simulated instrument time with these conditions and what regs would justify this:

VMC
Safety pilot on board
View limited device
VFR-only aircraft
Unapproved instrument navigation devices (ie. handheld NAV/COM)
Location USA

Say the pilot had a copy of Foreflight and a handheld NAV/COM with ILS capability. Aircraft is has typical VFR 91.205 equipment.

You are practicing LOC & VOR approaches? Don't those usually require a 2nd instrument as well to determine location (distance) fixes, such as the IAF?

I searched the airports around me at www.airnav.com, which also has links to FAA approach charts for each airport, and all needed two VORs, or VOR/DME, or LOC/VOR or LOC/DME. One VOR alone does not appear to be enough.

There seem to be a few radios with that capability  http://www.sportys.com/pilotshop/avionics-and-radios/aviation-radios.html  Reception range might be reduced, or spotty, without an external antenna.  You'd have to check on which had Marker Beacon reciever as well so you can tell how far down the approach you are. Have you done any searching at https://www.faa.gov/regulations_policies/faa_regulations/  under the 14 CFR Part 61 or part 91 sections yet for loggability?

If you interpret "under simulated conditions" liberally, I think that would satisfy the letter of the regulation, though not the spirit. You wouldn’t even need a radio with ILS capability since you can use Foreflight to fly GPS, Localizer, and VOR approaches.

Possible duplicate of Is it permissible to log an instrument approach for purposes of IFR currency in a non-IFR certified aircraft?

It is simply a VFR flight, you need to meet normal VFR requirements plus a qualified safety pilot to watch outside for traffic. I assume this aircraft has the minimum IFR flight instruments not including navigation, and is being used to practice basic instrument attitude skills, otherwise I don't know what the point would be. Flight by reference to instruments implicitly requires instruments to reference; a handheld device can not provide gyroscopic data as these inherently need to be fixed to the aircraft frame.

The question seems to be asking strictly about whether the regulations allow something, not whether something is possible.  As to what is possible, I've seen video of a Cessna 140 being flown in IMC with no artificial horizon, turn rate indicator covered up, wet compass as the primary reference instrument, pilot's hands off yoke, actively making course corrections with rudder.  A handheld GPS was being used for overall situational awareness to ensure that the aircraft was remaining in the general area where terrain clearance would be adequate.

"What is the optimal heading" for such a maneuver, could be grounds for another ASE question.  Most pilots don't know the answer.  Of course as per the actual question, one challenge with using the wet compass for simulated instrument flying, is that it is often located up near the windscreen.  However a "heading" display on a handheld GPS might serve as a feasible alternative.

Not sure what your comments are about. I answered about what the regulations allow, it is legally a normal VFR flight. A compass is an instrument. Handheld GPS does not provide heading, it provides ground track. Being able to fly with a wet compass if some of the gyros fail in flight is a regulatory requirement in the USA.

But the question was not "is it legally a normal VFR flight." The question was, may the pilot, wearing a view limiting device, log simulated instrument time. I took your statement "Flight by reference to instruments implicitly requires instruments to reference" to mean that it would not even be possible to control the aircraft (using a view limiting device) given the equipment list in the original question, and my comments were meant to suggest otherwise.

Re "Being able to fly with a wet compass if some of the gyros fail in flight is a regulatory requirement in the USA" -- since my comments, as well as the original question, referenced a case where no gyro instruments were available to the pilot, I'm not sure what the relevance of this comment is.

Sorry, would have been more clear if I had said the vacuum-driven turn rate indicator was the only gyro instrument aboard the aircraft (in the video referenced in my comment), and it was covered up.  Also, I just watched the video again and noticed the pilot was not using the hand-held GPS after all; it was only used to establish position before beginning the descent through the cloud.  So this event was all about the compass; hence my suggestion that under some circumstances blind flying (or simulated) would certainly be possible using the instruments listed in the original question.

(Bearing in mind that in many circumstances a hand-held GPS set to display a "heading" arrow, which is really the direction of the ground track, may outperform a simple "wet" compass as an aid to maintaining control of the aircraft without visual references.  Certainly this is true if the aircraft is being flown on a heading that is ninety degrees or more away from the heading that is the optimum one for flight by reference to the wet compass; the wet compass only works well over a very limited sector, where the "northerly turning error" is minimized or reversed.)


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How far should the two propellers on a twin engine plane be apart?
I was wondering if there are any specific ways to calculate the appropriate distance between two propellers for a twin engine twin fuselage planes like a Twin Mustang plane?

Welcome to the site. Your question is a bit brief and it is not clear to me what you are asking.

I was wondering about if there are any specific ways to determine the distance between two propellers for a twin engine twin fuselage planes.

If you want to determine the distance between two propellers, you could take out a tape measure and measure it.   If you want to calculate appropriate distance, it will be more complex.

Oh sorry for the unclear manner of presenting the question.Yes,i do mean if if there is an approximated way to determine the appropriate distance.

I'm assuming that you want to minimize efficiency losses due to propellers competing for clean air. I do no know what the optimal distance is but looking at a few practical examples it seems that as long as the propeller disks aren't overlapping you should be alright.

Here is a B 29 superfortress front view you can see the propeller disks are tangential to each other. The B 17 has a similar set up.

Here is a Tupolov bear as a more modern example there is more space between the propeller disks here than on the b 29. I would say this is probably because of the significant difference in horse power between the bears engines and a b 29's.
I hope this helps shed some light on what you were looking for. If you need more information your best bet is to look for a NACA paper on the topic. If there is research on this (and published in english) they are likely to be the ones who have done some.

Thank you very much for the information! It did answer my question.

For twin fuselage aircraft, the propeller distance is not the only parameter to look out for.
Safety-wise, the distance of propellers affect the size of the vertical tail and rudder. Because, the aircraft must still be trimmable / flyable in the loss of one of the engines.
The fuselage separation affects the weight breakdown of the aircraft as a whole. It is actually a relief for the wing loads.
From the viewpoint of Propellers, there's almost no problem in bringing them in close proximity. They can even intermesh, as in the case of some helicopters.
They will generate unwanted pressures to each other and extra noise, if they approach too much. However, if not overlapping, propellers do a good job of streamlining a flow (and adding a twist to it).

Thank you very much for the extra information regarding the effect of it.


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Why don't modern heavy bombers have gun turrets?
I know that for a period of time the B-52 had a tailgunner, but nowadays many heavy bombers lack any turret defense.Why?

Related: Why don't bombers have close-in weapon systems (CIWS)? and Why do heavy bombers not carry air to air missiles?

Related : U.S. Air Force Scientists are Working to Arm the B-52 Bomber with Laser Weapons

An interesting (although as it turns out, false) myth about bomber guns is about the tail gun on the B-58 Hustler. It was computer controlled, and used radar signals for data input. But, at supersonic cruise, the muzzle velocity of the rounds fired from the gun was (supposedly) less than the aircraft true air speed, so the bullets, although fired backwards at a approaching fighter, would still be travelling forwards when the target aircraft literally ran into them, from behind. Actually, the muzzle velocity of tail gun was 3380 ft/sec, and max speed of B-58 was 1600-1799 ft/sec.

You can only shoot what you can see, and today's enemy is often out of sight.

Also, current & foreseeable future enemies don't have jet fighters.

If they’re within gun range, you’re already dead.

There are really two reasons. First, fighter tactics have evolved over time, and second, the role of the B-52 has evolved over time.
Fighter tactics
Fighters used to depend (heavily) upon getting fairly close to an enemy, and shooting it down with a machine gun. Over the years, guns have become less and less dominant, and instead fighters tend to use missiles from outside a machine gun's effective range.
Role Change
The B-52 was designed from the beginning to carry nuclear weapons. Nonetheless, it's never been used to actually bomb an enemy with a nuclear weapon.
Early in its life, the B-52 was used to drop conventional bombs on enemies though. During the Vietnam conflict (in particular) B-52s were used to drop thousands of conventional 500 pound bombs on various targets.
The Linebacker II campaign was, perhaps, especially notable. During Linebacker II, 15 B-52s were lost--all of them to surface to air missiles (SAMs) not enemy fighters.
Linebacker II did see some limited success from tail guns as well: two B-52s shot down two MiGs. No B-52 (nor any other aircraft of similar size) has shot down an enemy fighter since.
Now, B-52s act as an aerial platform for launching stand-off weapons. Its normal operation is to fly in low (where it's difficult to detect on radar), and launch from a substantial distance from the target. Since it can be a substantial distance from the target, an enemy would need to defend a much larger perimeter (e.g., a large number of fighters basically blanketing a large area) to stand a decent chance of intercepting it.
Even if we assume an enemy fighter were to intercept the B-52, we get back to the situation with fighter tactics outlined above--they'd be unlikely to get within range of its tail gun anyway.
There are times the B-52's are still used for launching weapons at relatively close range, but only after achieving sufficient air dominance that doing so poses only minimal danger to the B-52s (where "minimal" basically means around the same level as training missions).

"an enemy would need a large number of fighters basically blanketing a large area to stand a decent chance of intercepting it" - or AWACS, or a target well inside their air defence region.

What do you mean by stand-off weapons?

@DavidGrinberg Stand-off weapons are missiles: weapons that you fire at a distance from your target and the weapon makes its own way there.

B-52s aren't just used for stand-off weapons: they've dropped a lot of dumb bombs in Afghanistan (and, as I recall, Iraq). However, that doesn't change anything of significance in your answer, since the Taliban has no aircraft and the coalition had total air superiority in Iraq, so guns on the B-52 wouldn't have been useful in either situation.

I was in the United States Air Force (USAF) working B-52's when the tail gunner position was retired. The reason was that it was deemed ineffective. As the answers here point out, fighter aircraft evolved such that medium and long range air-to-air missiles were the primary weapons. This kept fighters out far enough that the tail gun cannot hit.
But another very important reason was that the electronic countermeasures on the B-52s evolved such that it became the primary defense and was very effective.
Another major factor was that the fire control system was dependent on a radar transmitter/receiver. That in itself is a problem, because the radar signal could be detected and identified. When you are flying into enemy territory, you really don't want to do it with a radar transmission signal that says "BUFF OVER HERE WITH LARGE PAYLOAD THAT WILL DO MASSIVE STRATEGIC DAMAGE IF WE GET THROUGH".
It needed to be re-engineered to be more stealth, and the USAF didn't assess the time and cost to do that would pay off.

Because the defending nations stopped sending fighter planes at them to shoot them down at close range (where those guns were effective) and instead launched Ground to Air or Air to Air missiles at them from a larger distance.
This meant that it's more important to defend against missiles than against fighters.

@DavidK the fighters would usually make high speed passes through the bomber formations, firing their guns as they did so. They did get within dogfighting range, even if there was not much turning involved either by the fighters or bombers.

@DavidK there were dogfights between light and medium bombers and heavy fighters trying to intercept them in WW2. Think Me-210s vs. B-26s or Mosquitoes.

@jwenting Never mind, I think the edited answer is perfectly acceptable.

Gun turrets are for defending against aircraft that are within gun range. However, modern fighters will usually lock on and fire missiles long before the fighter is within the range of a gun turret.

Nowadays, you can lock onto planes that you can barely see or even not see at all. On the other hand, you have a very limited effective range with machineguns.
Also, keep in mind that shooting down a plane from another plane is very hard (check videos on gunner training in WWII). Now imagine trying to do so on very fast planes like nowadays fighter jets.
To sum things up, most of the time, you won't even be in range to be able to use your machine guns and, even if you were, it would probably be really hard to defend yourself effectively.
Keep in mind that adding these machineguns weights down the aircraft and might even hinder the installation of more important equipment like flares or jammers.
There was a tailgunner on the B-52 because the use of canons by jet fighters back in the Vietnam war was more common than nowadays.

A WW2 20mm and/or a .50 cal machine gun fired at 600 rounds/minute and was human-aimed; a modern CIWS fires at 4000 rounds/minute and aimed by radar and computer ... not very comparable IMO.

That means you need an engine for the turret, you need the program to aim the gun (which might require a more expensive computer) and you need to pack up enough ammo to back up these 4000rpm. All of these problems make the plane either more expensive and much more heavy, all of that for a mere novelty.

They put a canon on modern fighter planes (e.g. the F-35), I wouldn't have expected they'd be too weighty for a modern heavy bomber.

Yes, but that still would be a novelty. It's already more or less a novelty on modern fighter planes but at least they can do some strafing run to suppress some targets. The weight you don't use for a tail canon can be used in flares, jammers or something more useful and relevant.

Modern fighters are usually armed with missiles, with which they can engage from far beyond the range of guns. At least one North Vietnamese fighter was shot down by a B52D's rear gun armament during the Vietnam War, however.

I believe that another reason is to do with the speed of modern bombers. At transsonic speeds old-style gun turrets would be ripped off the plane.  On helicopters and other non-transsonic aircraft one can still find modern equivalents of the gun-turret. Have a look at e.g. the  Mil Mi-24 https://en.wikipedia.org/wiki/Mil_Mi-24

B-52s had tail gun turrets until 1994, so this can't be the reason.


text:
Do airlines use a software to calculate center of gravity?
How do airlines calculate center of gravity? I have read in several airlines websites that they optimize the center of gravity position, to improve aerodynamic performance and reduce fuel consumption. Do they use some kind of software to do this?

Related: How does a commercial airliner measure its weight/mass? and Do additional passengers impact on weight and balance of a domestic airliner?

Most airlines of any size use software to calculate the c.g., either their own software or vendor software.
Passenger aircraft weight & balance is relatively trivial compared to cargo aircraft w&b, where such things as floor loading, lateral imbalance, and monocoque limits can come into play.
http://www.champ.aero/ is one provider of that service and software for cargo aircraft for those companies not wishing to program their own.
Fuel efficiency is one advantage of loading to given c.g. An oft-used method is to load to a specified zero fuel weight c.g. For example, on 747 aircraft through the -400 models, a zfw c.g. of 26.6% mac is a typical target.
If you want to see and play with a free, online, open-source w&b system for cargo aircraft, go to http://terryliittschwager.com/WB/index.php.

Airlines are allowed to calculate center of gravity by using legally defined assumptions for what the weight of each person will be. It might vary between countries, but normally there are numbers for men, women, children under 12, and infants up to 2 years old. These weights can be added to in the winter to account for extra clothing in some regions. All the information is sent into a computer thats then says if they are a go or not.
In theory there are much better was this could be done, for example adding a weighing system to each leg of the landing gear would allow precise control. In the end though things move around in flight and the baggage handlers aren't exactly known for their careful placement of luggage, best you will probably get is put more stuff that way.
In flight would seem a little trickier, however most aircraft record data such as angle of attack, flight speed, ambient pressure, and control surface position. Using a spreadsheet generated from models, an aircraft could easily determine its approximate cg by comparing how much lift it should be producing, where the center of pressure on the wing should be, and how much the horizontal stabilizer is pushing downwards.


text:
How does an engine driven vacuum pump used in gyroscopic instrument work?
In general aviation light airplanes (C152,C172,PA28..) how does the engine driven vacuum pump work?How is it made? and if the filter of one of the gyro instrument is clogged would the vacuum indication still in the green arc? (The latest question bother me the most!)

Have you tried any of the informative and free Handbooks available from the FAA?

Not really. I'll have a look, thank's. I'll see if easa has something similar as well.

Related info here

It uses a system of vanes to draw air through the inlet port and expel it through the outlet port. The engine's lubricating system provides oil for lubrication as well as sealing the internals. The inlet is connected to the instrument and this provides the vacuum the instrument needs. To answer your last question, if the instruments are connected in series and then to the vacuum pump, and one of the instruments obstructs the air flow in the system, you may loose vacuum and instruments can become unreliable or stop working.


text:
Where do I find a complete Volmet frequencies list for Europe?
Is there any website or ,even better, book to buy to get such frequencies?

VOLMET frequencies are published in the national AIP's. To my knowledge, there is no single official resource

List of HF frequencies for VOLMET worldwide.

@ymb1 although, to be honest, we have a couple of well accepted resouce location questions (including one of mine) we probably have to decide if to make them "historical" (and this deserves a meta post)

I have a list, you can download it here VOLMETS.I would really appreciate if you can help me confirm stations that are operational. Obviously depending on your location, receiver and antenna you will not receive all these.
The list is by no means 100% accurate and there may be stations that have ceased operations or are very sporadic in operation. Without others assisting it is impossible for me to verify each and every station.

You can look it up on this link world wide also for europe.
VOLMET


text:
Can a TCAS be alerted when the intruder aircraft has a transponder in stand-by mode?
Can a TCAS give a TAs (traffic advisories) if the intruder aircraft has a transponder in stand-by mode?

No. If the intruder's transponder is in standby it will not respond to interrogations and will be invisible to the TCAS. TCAS can't act on targets it can't interrogate.

A TCAS in standby mode still replies to discrete interrogation. What would happen if TCAS1 had already registered the presence of TCAS2 before  TCAS2 was switched to standby mode?

@mins It might work if the intruder was close enough to be put on the track list before the transponder was set to standby. TCAS can only issue a TA or RA when the intruder is actively interrogated. But even then, if it slides off the track list and it's in standby, the TCAS won't re-acquire it as there won't be a short squitter to announce it. Going forward, if TCAS2 has ADS-B out and TCAS1 has hybrid surveillance/ADS-B IN, it would be possible to see the aircraft on the display. but without a TA or RA.


text:
Why on weather surface charts high pressure is blue and low pressure is red?
Is there any relation with the temperature of the air or is it just a standard procedure?

This is probably a better fit over on the Earth Science SE community.

The red color is universally associated with evil, suffering, blood, fire...
Low pressures are associated with rain, storms, even with hurricanes and tornados... High pressures are associated with stable weather, sun, blue skies, absence of wind... Hence, it's no wonder that low pressure is indicated in red, and high pressure in blue...
Socialist-minded people do strongly object... For them, the red color is a symbol of revolutionary progress...

Well, we're told " The people's flag is deepest red: It shrouded oft our martyred dead", so I'm not sure socialist-minded people would actually disagree: they see a red region just see a tornado to chase or a hurricane eye to fly into, ;-) .

Red is also the color of love, passion and happiness. And in China, Red, corresponding with fire, symbolizes good fortune and joy. (Wikipedia).

"From high to low, look out below."  Pilots use this mnemonic to remember that when flying from high pressure into low pressure, the altimeter will read higher than actual altitude, which can be dangerous. (also true from high temperature into low temperature).
I don't know if it was by design, but I've always associated the red low pressure with danger, because flying from an area of high pressure into an area of low pressure can result in flying lower than indicated on the altimeter and also, low pressure is usually associated with less than desirable flying weather. Additionally, high pressure is often associated with blue skies. So maybe that is the rationale, if there even is one.

The colors of high and low pressure areas are chosen to represent what occurs aloft, not at the surface. Lows are composed of warm rising air; highs of cold sinking air. That's why highs are represented by blue, which equates to cold air, and lows are represented by red, which equates to warm air. Again, this is aloft, near the top of the troposphere, not at the surface. This website explains the sinking and rising aspects of highs and lows.

Wouldn't it be an appropriate analogy to equate low pressure with lower frequency light color (red) and high pressure with higher frequency light color (blue)?  Also, more energy in higher frequency wavelengths (blue) than lower (red)...


text:
What is the difference between frequency wild AC system and other AC systems?
What is the difference between a standard AC system and an "AC wild" system? How does the latter work?

Welcome to aviation.se.  I've never heard of "wild AC" --- can you provide a link to some example of that term being used in context?

Related: Variable frequency generator on A380, and why AC generators are moving from fixed to variable frequency.


text:
In high-altitude piston aircraft, what type of forced induction (boost) is used?
In high-altitude piston aircraft, what type of forced induction (boost) is used? Turbocharger or supercharger?

Maybe you should rephrase your question using commonly understood terms, like supercharging for a compressor which is directly driven by the engine and turbocharging where the exhaust stream drives a turbine which in turn drives a compressor.

@PeterKämpf edit my edit of the question, or the original user who posted the question?

Initially, all supercharging was done with, well, superchargers. Read NACA Technical Note 48 for a nice summary of German work on superchargers during WW I which reached the point of almost being ready for operational use when the war ended.
From Notes on WW I German superchargers:

All of these projects failed in the sense that the war ended before
  the technology could yield military advantage. While we have nothing
  by way of the detailed reporting on German wartime research accorded
  to British and American efforts, it appears that Germany came closest
  to producing combat-ready supercharged engines. Certainly they had a
  denser supplier network than the Allies, more consistent research
  goals, and accumulated far more hours of test flying.

Turbochargers were investigated soon after but were used much less frequently for two reasons:

Due to the higher inertia a turbocharger reacts only slowly to speed changes, which limits the pilot's options especially in fighters.
To avoid overstressing the engine, turbochargers need a wastegate. Before automatic wastegates were developed, the risk of manual mishandling of the wastegate setting was all too real.

From Wikipedia:

Turbochargers were first used in production aircraft engines such as
  the Napier Lioness in the 1920s, although they were less common than
  engine-driven centrifugal superchargers.

Only when bombers and airliners needed to maximise their range did the advantage of turbochargers, their higher efficiency, take effect. In many cases a supercharged engine had a turbocharger added because the high energy content of the exhaust gases offered easy gains in efficiency. Note that most early examples of turbocharged aircraft were types in which the engines are run at a constant power setting over many hours. What also helped was the development of jet engines which spurred both the research into high-temperature alloys and turbine aerodynamics. This was a mutually reinforcing development, because the first turbojet designs were based on then-existing turbochargers.
With automatic wastegates, little requirement to frequently adjust engine power, much experience in high-temperature materials and high fuel prices, today the turbocharger is the obvious choice for boosting engine pressure. The most recent piston-powered high altitude platforms, the Boeing Condor and the Grob Strato 2C, used turbochargers. In case of the Condor this was a two-stage turbocharger with intercooling, however, flight test revealed synchronisation problems between both stages. The Strato 2C had two turbochargers and one supercharger, all with intercooling.

WWII bombers needed forced induction to achieve performance at ALTITUDE - NO other reason!
Numerous WWII fighter aircraft relied  on supercharging and turbo charging to improve overall performance especially at higher altitudes…. Such as the P-47 and the P-51….

This is really a comment, not an answer to the question that was asked.  You'll need to build reputation points with quality answers before you are able to comment.  This one probably won't get you there...


text:
Which antenna on an commercial airline airplane is used for the transponder (SSR)?
On the 767-200 image I see no SSR antenna, but since that TCAS uses the transponder I guess that are sharing the same antenna. Am I right ?]1

Short answer
The SSR antennas are labeled ATC L and ATC R (two at the top, two at the bottom).

Source
Both SSR and TCAS systems use the same communication protocol and use the same frequencies (1030 and 1090 MHz). They could well use the same antennas. However for best performance, SSR antennas are omnidirectional and TCAS antennas favor some sector, usually by beam forming.

On this B767-200 picture, TCAS antenna is the flat one at the front, and the yellow SSR antenna is a bit behind (only one):

See the full and higher quality photo by Ramon Jordi
Another airframe with two SSR antennas:

See the full and higher quality photo by Temo Madrigal
I was not able to find a good view of the SSR antenna on a B767-200, but on this A320, the two side yellow SSR antennas are more visible (as well as the single DME antenna behind):

A320 SSR antennas, source

Use and needs are different

I guess that are sharing the same antenna. Am I right?

As said, you could be right as SSR and TCAS operate on the same L-band frequencies, and can't time overlap. But they have actually dedicated antennas, because they need different radiation patterns:

Transponder initial function consists in responding to SSR interrogators on the ground, and now in broadcasting ADS-B squitters in addition. The bottom SSR antenna is of some omnidirectional type, commonly a quarter wavelength monopole), Because the fuselage is not a perfect ground plane, the radiation pattern doesn't peak at the horizontal as one would expect, but has a privileged direction at 30° (source) below the aircraft plane, a low gain in the horizontal plane, and is nearly blind on its nadir.
TCAS on the contrary need to have a good reception in or close to the horizontal plane and around the nadir. They are the directions where the most immediate threats are coming from.

This is why the TCAS antenna is designed differently. Let's see the backside of the TCAS antenna shown earlier:

Source
Hum... not one antenna connectors, but four!
The flat shape of the TCAS radome (sometimes round) contains a 4-element phased array antenna which can be set to cover an hemisphere, but which can also focus on a quarter of this hemisphere. Example of design using a switched beam-forming network (SBFN):

Source: Microwaves & RF
The four outputs labeled A1..A4 are connected to the four individual connectors under the radome.
TCAS uses the steering obtained with the array to select which direction to send interrogations and to listen to responses, in order to reduce synchronous garble from close mode C transponders responding simultaneously to the aircraft (1.7 NM radius), and FRUIT from all aircraft responding to a ground ATCRBS interrogation.
TCAS reverts the bottom antenna to the omnidirectional mode (same phase on the four antennas) and decreases RF power in landing and takeoff phases, to prevent radio spectrum overload.
For a Mode S transponder aircraft, we would have this kind of configuration:

ACAS II Guide
Note there is a connection between TCAS and SSR to inhibit each other when they are using the frequencies and protect their RF input. If I remember well, SSR mode has the priority (at least when the TCAS is not in a TA/RA phase).

Collins Aerospace developed an integrated system for the B787. The ISS-2100 combines TCAS, XPDR, WXR and TAWS. The WXR and TAWS only share packaging and I/O with the TCAS/XPDR which is truly integrated. The integrated TCAS/XPDR is now available as the TSS-2100 (without the WXR or TAWS). The integrated TCAS/XPDR operates with just two specially designed antennas. They are a hybrid design that supports both omni (for XPDR) and directional (for TCAS) operation. So if you check out B787 you will find just 1 top and 1 bottom ATC/TCAS antenna. The 2 installed ISS share the antennas via relays.

@Gerry. Thanks, this is an important information.

@mins The purpose of Mode S transponder antenna is to respond to interrogations from SSR (Ground) and ADS-B Squitters. It is understandable that placing the antenna at the bottom of the aircraft serves the purpose of communicating with the ground antenna well, but what is the purpose of placing the Mode S transponder antenna at the top ?

@KrishnaKanth Reason for top transponder antenna: At least 4 reasons: 1/ On the ground, the bottom antennas are ineffective, so one antenna must be elsewhere for SSR and ADS-B transmissions. 2/ Space-based ADS-B, 3/ To answer ACAS interrogations coming from above, because ACAS delegates 1090 MHz transmissions to mode S transponder (more) 4/ Even without TCAS, mode S is still capable of replying to ACAS interrogations, which may originate from an aircraft above.


text:
How (and when) to use the BFO button on an ADF?
BFO (Beat Frequency Oscillator) is a function of the ADF, I know that it has something to do with different types of NDB, but I do not know which type. And how and when do I use it?

I heard that depends on the type of emission of the NDB in use. For what I know there are N0NA1A or N0NA2A but I do not understand how this imply the use of the BFO push button or even the differences (from operational point of view) between the two!

Let me try to explain this from the radio perspective.
In a regular AM transmission (A3E emission type), you have a carrier and two side-bands. The usual receivers use the carrier to demodulate the audio information in the sidebands. The spectrum looks something like this:

Note: Horizontal axis is frequency, vertical axis is amplitude.
In real-life, it would look something like this:

Note: Vertical axis is time, with present being a the bottom, horizontal axis is frequency, increasing from left towards right and amplitude is encoded in color, with dark blue/black being low amplitude and white being high amplitude.
The two sidebands are the wavy-looking structures to the left and right and they carry the audio information. The carrier is in the middle and can be seen as the bright white line, especially visible during moments of silence.
When we want to send let's say Morse Code telegraphy over AM, we get the A2A emission type: Carrier and two sidebands, each carrying Morse code data.

Or in real-life something like this:

Here, we can also see the bright white carrier and the Morse code in the sidebands, with lower being a bit weaker than the upper.
Now, our AM receiver doesn't really know what's in the sidebands, the audio just goes to the speaker. So no special adjustments are needed to use a AM receiver for A2A emissions.
OK, so what does this have to do with BFO? Well, BFO is needed when we want to receive carrier-less data using regular AM receiver.
Let's now take a look at the spectrum of a A1A signal:

Or in real-life something like this:

There isn't much to see, only the "carrier". However, if we take a look at a real-life recording of the spectrum, we'll see that the carrier is being turned on and off, with no sidebands.
Now, when we input that to our AM receiver, we won't hear the Morse code. Depending on the receiver type, there might be an audible lack of noise when the carrier is on, but the expected tone is not there, since the sidebands are empty.
The BFO (Beat-Frequency Oscillator) helps us there. It artificially re-inserts the carrier, so combination of the re-inserted carrier and the A1A signal reach the receiver. The receiver thinks it's hearing a regular A2A or A3E signal and just outputs the received Morse Code tone. On the spectrum, we see something like this:

So the answer would be to use the BFO with A1A NDBs and not to use it with the A2A NDBs.
Possibly useful extras:
Why the name BFO?
When two sinusoidal signals, with frequencies A and B are mixed together, the result is a pair of new signals with frequencies A+B and A-B. This phenomenon is called Beating. In case of radio, if the signals are close together, the difference can drop down into the audio spectrum. For example, beating of 353 kHz signal with 352 kHz signal will result (among other things), in a 1 kHz signal, which we can output to a speaker.
A3E/A2A power and spectrum efficiency:
The carrier and one sideband do not participate in the actual transmission of information and are therefore wasted power. Newer systems try to avoid using them, when possible, also saving space in the spectrum. However, the carrier simplifies receiver design.
A2A variants:
Depending on the region, the A2A beacons might have the carrier on all the time, as shown above, or might key the carrier as well as the sidebands. Furthermore, some might only have one sideband, and save some power through that. Regular AM receivers can receive signal with a carrier and only one sideband. This is called Compatible Sideband.
J3E and BFO:
J3E, aka Single Side Band is a refinement of AM which is mostly used on high-frequency bands. It eliminates the power wasted in the transmission of the AM carrier and the other sideband and also uses less spectrum space, which is in shortage on the high-frequency band. On the other hand  the receiver and transmitter are more complicated. In aviation, the upper sideband is currently used. An AM receiver with BFO can be also used to receive J3E transmissions. Here's a spectrum of that:

A2A and BFO:
Not  coming from the aviation field, I don't know how the exact settings on the instruments work, so some of these might not apply due to precise offsets and audio filtering, but in general, if you re-insert the carrier on an A2A system, one of the following will happen:
The reinserted carrier frequency matches the original carrier. In this case, you won't hear any difference.
The reinserted carrier has a slight offset from the original carrier. In this case, beating will occur. You should hear the Morse code identification properly, but you might also hear a low-frequency sound in the background, if the system is capable of reproducing it.
The reinserted carrier has significant offset from the original carrier. In this case, you'll be able to hear both the original carrier and the Morse code identification. Note that the audio frequencies might be shifted in unusual ways, depending on where exactly the reinserted carrier was actually reinserted.

People with aviation backgrounds might not be familiar with waterfall displays.

@rclocher That'a why I added a written explenation below the first waterfall. If you think it's unclear, please let me know, so that I can improve it.

Very nice answer, well explained.

Very nice answer. I was not really familiar with the material but managed to understand the point of it well enough. The waterfall diagrams were well-explained (and also quite intuitive) in my opinion.

excellent answer

Beat Frequency Oscillator (BFO):

To make the unmodulated parts of the A1A signal audible, ADF incorporates a BFO.
This produces a signal slightly removed from the received frequency that is then mixed with it.
The mixing of the two frequencies produces an audible beat frequency, the difference of the two.
e.g. a BFO would generate a signal of 402 KHz to match a received signal of 400 KHz.
The mixing or heterodyning of the two produces signals at 400 KHz, 402 KHz, 802 KHz and 2 KHz .
The beat frequency of 2 KHz is the audible tone.
Selecting the BFO on makes the N0N carrier wave audible and allows the A1A type of ident to be heard.
BFO is not needed for an A2A signal which is already modulated to an audible frequency.

Source: theairlinepilots.com

Any improvement will be appreciated. @ymb1


text:
Does an heavy propeller aircraft (CS-25) have the Blu Line (Vyse) displayed on ASI?
Does a propeller aircraft that is categorized as CS-25 (EASA Regulation) have the Blue Line (Vyse) mandatory displyed on ASI?
I know that would be the case for lighter aircraft (Less than 19000 lbs /8600kg), but my question is related for heavy aircraft (Dash8 example).

No, larger aircraft typically don't have such speeds shown on the ASI because they vary significantly with weight. See also Why don't Transport Category aircraft have a listed Vy?.
For the Dash-8 Q400 for example, the $V_{\text{y}_\text{SE}}$ speed is called $V_\text{CLIMB}$:

VCLIMB – Single-engine best rate-of-climb speed.

The following table shows the values for $V_\text{CLIMB}$ as a function of weight:
+-----------+--------+
|  Weight   | VCLIMB |
+-----------+--------+
| 18.000 kg | 130 kt |
| 20.000 kg | 131 kt |
| 22.000 kg | 137 kt |
| 24.000 kg | 143 kt |
| 26.000 kg | 148 kt |
| 28.000 kg | 154 kt |
| 29.000 kg | 158 kt |
+-----------+--------+

As you can see, the values vary too much to show a single blue line on the ASI.
This does not mean that the speed cannot be displayed at all. The Dash-8 Q400 has a Primary Flight Display (PFD) and can show up to 5 speed markers on the speed tape ($V_1$, $V_\text{R}$, $V_2$ and two more called #1 and #2, typically used for $V_\text{FRI}$ and $V_\text{CLIMB}$ respectively during takeoff and initial climb).

(Dash-8 Q400 FCOM - Indicating and Recording Systems)
The resulting markers on the PFD look like this:

(Dash-8 Q400 FCOM - Indicating and Recording Systems)
The electronic standby instrument does not show these markers.


text:
What is the minimum separation for navaids to work properly without interfering?
What is the minimum separation between navaids (for VORs/DMEs and NDBs) for them to work without interfering, by regulation and by functionality?

Do you mean navaids using the same frequency, or different frequencies?

Interferences are possible, and regional meetings are used to determine frequencies and power used. Such rules are found in Annex 10 of ICAO Convention, e.g. "On the assumption that a VOR having an ERP of 17 dBW (50 W) is to be protected to a service distance of 46.25 km (25 NM) and an altitude of 3 000 m (10 000 ft), protection of the VOR system requires that the ILS be at least 148 km (80 NM) from the VOR.". For NDB: "A minimum protection ratio (wanted/unwanted signal ratio) of 15 dB is to be used as the basis for frequency assignment planning". This is a broad topic.

VOR/DME and NDBs can't interfere with each other: they work on completely different wavelengths, so you could have a VOR right next to an NDB and they'll work just fine. Similarly, VORs on different frequencies won't interfere with each other; you could have them in close proximity as well. The only time you are likely to see navaids in close proximity is at an airport, which may have an NDB, VOR/DME, and multiple ILS in operation (an airport with 2 airstrips may have 4 ILS in operation: 1 per runway).
Where you do get interference is when stations are using the same frequency. A VOR has a service volume, a set distance it is designed to reach; in the US the high altitude service area is a minimum of 130 nautical miles at 45,000ft, but in practice in the right conditions you can pick them up further than that. Some stations are set at a lower power and have less range.


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How to glide in a circular pattern?
I was just curious as to how you would make a glider glide in a circular pattern ( or helical ).
Will there be a large difference when considering a conventional glider vs a delta wing glider?

Assuming there is no wind, a circular pattern is most easily achieved by flying a constant speed and a constant bank angle. Maintain coordinated flight (no  skidding, no slipping). There is no difference in turn dynamics between a conventional glider vs a delta wing glider.

Delta gliders have a unique way of turning amazingly similar to the wing warping technique attempted in the early days of flight, using DRAG differential to yaw the aircraft.
This is the result of the differential of sweep angle with a change in relative wind, between the inside and outside wing.  Elevons are used to roll the plane to its bank angle. The resultant sideways motion creates more drag  on the inside wing, more than the "adverse yaw" created by the elevon deflection.  The plane will turn without a rudder.  Examples of rudderless deltas include hang gliders (weight shift) and flying wings (elevons).
Straight winged gliders do not create sufficient yaw drag differential when rolled, in fact, the ailerons will tend to yaw it in the opposite direction, hence the need for rudder input.
Conceptually,  the rudder is the inside wing for a delta, and the tail for a straight winged aircraft, with many possibilities, including spoilers, in between, depending on your design style.
The large trailing portion of the delta also contributes to the turn.

Yaw from differential drag isn't unique to deltas, btw.  It's also done by the spoilers of the B-52.

A conventional 360 turn can be flown the same way in a powered airplane and glider: a gentle bank towards the turn, using rudder to maintain coordinated flight.
However, the purpose of a glider circling is often to catch a thermal (i.e. lifting air). In order to maximize the lifting force, a glider pilot might keep the wings level and use rudder to turn, a technique that is unique to gliders.

"In order to maximize the lifting force, a glider pilot might keep the wings level and use rudder to turn, a technique that is unique to gliders." – How does that work? Surely your goal in a thermal is to minimize your sink rate, and surely the way to minimize your sink rate is to stay coordinated. Maybe I should post a question about this.

In basic glider training, I was taught to remain coordinated when circling in a thermal.  I don't know if a different technique is used by experienced pilots, but for me the student, the instructor insisted on the yaw string being aimed at my nose.  Circling in a thermal is done pretty close to stall speed, and being uncoordinated would increase the risk of a stall/spin, so it seems like a good idea.


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Does the power behind the engine make any difference to how strong the thrust is?
What difference does horsepower make? If the engine can spin the propeller fast enough, why does it need power behind it?

I feel like there is a similar (but not duplicate) question around here somewhere.  If I recall, it answered the question title quite well.

It wouldn't make any difference in vacuum and with perfect bearings.

@dalearn Maybe this one: https://aviation.stackexchange.com/q/43121/18733

@PerlDuck yes, indeed.

to understand this, just put your hand in a bath of water and try moving the water around quickly.  it surely takes some power!

If the engine can spin fast enough it must already have enough power to do so.

@EricDuminil A propeller in vacuum would be quite pointless.

@Mast sure. And this fact should help to answer the question.

This is not a duplicate of the other question. The answer may rely on the same concepts as the answer of the other question, but it doesn't make them a duplicate, and I doubt very much that the answer to the other question will help resolve this question for the person who is asking it.

I'm voting to reopen. If someone doesn't know that you can't produce RPM without power, then saying "you can't produce power without torque" isn't going to help.

You ask:

If the engine can spin the propeller fast enough, why does it need power behind it?

Good question. However, the answer is in the if of your question.
A propellor does work - it pulls (or sometimes pushes) the aircraft through the air. In order to move the aircraft at a useful speed (or even at all) it has to spin pretty fast.
The faster it spins, the more work it has to do. That work requires power. If the engine doesn't have enough power, it can't spin the propellor fast enough.
So yes, though the power and the speed of the spin are not the same thing, you won't get the spin without the power.
If you consider a car, the engine has to spin the wheels fast enough to move it along at the desired speed. And what makes it possible for it to spin the wheels fast enough is power. Otherwise, the wheels won't turn fast enough (or at all), and the car won't move fast enough (or at all). It's the same in the case of an aeroplane.

The key here is that if you lift the car off the ground, it takes almost no power to spin the wheels as fast as you like. It's easy for someone unfamiliar with the physics of it to think that it must be similarly easy to spin the propeller - after all, it's about the same size, right? The difference to note is that the propeller is designed to push a lot of air while spinning, while the average car wheel experiences relatively little air resistance against its rotation. A fairer comparison is to put the car back on the ground so that the wheels have something to push against.

Maybe a boat with a prop is an easier to understand scenario?

Consider spinning a flat disk, for instance the platters in a disk drive.  Takes very little power to turn them at high speeds - 7200 RPM or more - because they're not pushing air around.  If you take that disk and make it into blades, like a simple household fan, it starts pushing air, which takes work.  Propellors are designed to push a lot of air, so it takes a lot of work to turn them.

@anaximander Why on earth would anyone lift a car off the ground?!

@DanieleProcida: A mechanic in a repair shop using a car lift?

@Jonathan Or anyone else changing tires. They don't last forever, after all.

You don't even need to lift the car off the ground, just try starting on an icy road.  Especially if you're pointed uphill :-)

Most propeller-driven airplanes use constant-speed props. That means they use a governor to vary blade pitch (and thus resistance) so the engine always spins at the selected speed.
The more power you have turning the prop, the higher the blade pitch will go, biting deeper into the air and thus generating more thrust, and therefore the faster the plane will travel.
The main exception are very low-powered aircraft, such as used in primary flight training, which can barely keep the prop spinning at a decent speed even at a very low, fixed blade pitch.

"the more power you have, the higher the pitch will go" wouldn't it be better to swing your prop at optimal AOA, and match its size and the engine properly?  Lots of power tends to "spoil" engineers, we should not forget efficiency.  "Very low-powered aircraft" are speed envelope limited due to their fixed pitch, but a larger, slower variable pitch prop with produce more thrust for a given amount of power because "moving a lot of air a little" more efficiently generates thrust per horsepower.

@RobertDiGiovanni Prop size is limited by ground clearance and tip speed. At some point, you have to add blades to keep the prop AOA below stall, which then introduces new problems.

The Wrights had a pretty good idea adding two 2 bladers to 1 engine rather then 4 blades to one prop.

A prop is just a wing going in a circle.
If you have a wing going straight through the air, you need force propelling it along, a thrust source, and that force is used to make the wing redirect air down by operating at an angle of attack, creating lift, which is vertically oriented thrust.   It takes energy to do this.  In a single engine airplane the wing is being driven forward in a straight line by a thrust source on the fuselage.  Unless it's going downhill, it has to have the thrust source.
A prop, or helicopter rotor, is the same wing being driven around a central axis instead of linearly.  Being driven around an axis, instead of linear force being applied to the wing, it's rotational force, or torque, which comes directly from the engine.
In the end, it's still force applied to generate aerodynamic thrust forces, but in the prop's case the output is horizontal thrust instead of a vertical thrust force (lift).
So in an airplane, the stronger the thrust force pushing or pulling the plane along, the higher angle of attack the wing can operate at for a given speed, driving air down harder, and more lift (vertical thrust) is created.  Or it can stay at the same angle of attack and just get pushed through the air faster.
With the propeller, the stronger the rotational force, the higher the propeller blade's AOA can be and/or the faster it can go, driving air aft harder, and more thrust (horizontal lift) is created.
So the more propeller thrust you want to make, the more horsepower (torque x velocity) is required.

Because spinning the propeller to make thrust creates friction and drag.  Using a piston engine as an example, when we start the motor and add fuel, the motor will spin the propeller faster and faster until the friction of the pistons moving, plus the energy required to move fuel/air mixture in and exhaust out, plus the friction of the bearings, plus the drag of the propeller through the air, equals the expansion force released by the fuel burning in the cylinders.
Once maximum rpm is reached, this is called "steady state".  The average force of the fuel being burned and pushing the pistons is equal to all friction and drag forces.  If you designed your engine well and it does not overheat, you get maximum THRUST until it runs out of fuel.
So for a given prop, the more horsepower you have the more rpms you get.
Sizing props with engines can take some time to learn, but it is generally better not to overspeed the engine by trying for very high rpms and risk overheating it.  A larger prop swung more slowly will be more efficient, but you still need to burn enough fuel per unit time to get it going.

I'm no an expert in aeronautics or fluid dynamics, but I'm pretty certain that the "friction" aspect of it is pretty nearly trivial. It's the lift induced drag that's the main factor. It takes power to turn the prop because the turning prop moves a huge mass of air.

@Solomon Slow I recently learned at higher rpm, pumping the fuel/air in and exhaust out of the engine actually becomes significant, and piston/bearing friction also take a part of the power pie.  It gets interesting to "load" the engine with various props to get optimal performance.  Pitch and prop length can be tested.  For example, a 10/5 or 10/6 prop may work very well for takeoff, but a 10/7 is better in cruise.

OK, I wasn't thinking about friction in the engine itself. I saw "horsepower" in the question, and I only thought of shaft horsepower.

Newton famously (is quoted as saying) "For every action there is an equal and opposite reaction.
Propellors propell by forcing air in one direction (action) and in turn generate a force against the propellor (reaction) which drives the air forward.
The classic drag equation
Force = 0.5 x Air_density x Cd x A x V^2   ... (i)
&  the related
Power = Force x V ... (ii)

0 < Cd <= 1 is the drag coefficient. Cd=1 is the best case in this application.
A = area (of prop circle in this case),
V = velocity of air through prop,

when analysed these equations are found to link the thrust, and the power imparted  to the air which passes through the propellor.
ie to get air of a certain velocity, or a certain amount of thrust you need to 'plug in' appropriate figures into the above equation. As soon as the propellor starts moving air it will generate thrust and thrust is linked to the power consumed (or delivered) by the 2nd equation.


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How do planes maintain constant speeds at cruise altitudes?
I'm a noob so pardon my ignorance. So my understanding is that as the plane gets lighter during the flight, its mass reduces therefore reducing the lift needed to maintain the altitude. At those altitudes and speeds, my understanding is that we can't engage slats or flaps. As a result, we would have to reduce thrust to reduce the lift to account for the loss in fuel and maintain the same flight level
So what can one use to continually reduce lift coefficient at those altitudes to maintain constant airspeed and flight levels? I understand that ATC also clear aircraft to higher FL thereby reducing the air density and therefor reducing lift and enabling higher airspeed but that still leaves us with the problem of flying at constant airspeed at cruise altitudes.

One word: autopilot.

Note that for this reason (and because it flew higher than other airplanes so it didn't have traffic to avoid above it) the Concorde did climb during cruise.

@ManuH Concorde was also an extreme design that was atypically (for a passenger jet) susceptible to huge efficiency spoiling by just about anything that deviated from its ideal flight configuration.    A normal subsonic tube jet is not punished so severly for maintaining a fixed pressure altitude during cruise, so really the reason that Concorde did not was more to do with its particular and highly constrained flight parameters than anything else.

@JonathanWren: Some of it's mass is always going out the back of the engines, though (burnt fuel). That's far more significant than any change in gravity due to altitude.

In fact the difference due to additional height only lowers the force due to gravity by about .33%, and as @FredLarson says, is completely negligible when compared to the mass change due to fuel losses

@FredLarson Oh, ha! Very good point. My mistake.

Notice you ask about speeds in the title but then it goes about lift. The speed should stay the same as long as the altitude stays the same, because the shape of the plane is the same [see: Galileo, falling bodies in vacuum, etc]... So if the autopilot (only) keeps it on the same level height with the same engine settings, no speeding up happens.

The autopilot pitches to hold the flight level when it captures the level at the top of the climb, so later on as the aircraft gets lighter and wants to climb further, the A/P will lower the nose as required to hold the flight level (the A/P is able to move the elevator through its servo's link to the elevator controls; it can also work the trim if the servo has to hold too much out-of-trim force for too long, to avoid wearing out the servo - in effect it does what the human pilot does, more or less).  It happens very slowly and significant amount of pitch over is not apparent until some time has passed.
Effectively, AOA is being reduced by the A/P to compensate for the reduction in wing loading.  Because induced drag is being reduced, if thrust is not reduced to compensate for the drag reduction, the A/C will accelerate.  If your clearance requires you to maintain a certain Mach #, this is a problem.
If the jet has an autothrottle system that can be configured to hold a Mach # while the A/P holds the altitude, the thrust is adjusted automatically.  If not, the pilot monitoring at some point will notice the Mach # creeping up, and will reduce thrust in very small increments compensate and maintain the flight planned/cleared Mach #.

Which control surfaces does the autopilot use? I guess the elevator? This is probably obvious to pilots but maybe worth adding here but with this being an elemantary question ... after all, the autopilot can't "think the plane" level! :-)

Thanks for the answer John. So the A/P is lowering the nose by moving the elevators? Is that a correct reading of the problem? But the other question I had was, by lowering/raising the nose, aren't you also changing the direction of the engine thrust? It just seems a little counter intuitive to me that the A/P would lower the nose. May be you could throw a little more light.

Text added thanks.  The other thing that happens, much more suddenly, is you will fly into some mountain wave that is rising maybe 50 or 100 fpm, and you will observe the nose pitching down on your PFD because the A/P wants to hold the alt and  starts applying down elevator, and you have to reduce thrust maybe a percent or so to keep the same speed.  Then you fly out of the wave and it pitches back up and you have to put the thrust back the way it was.

@SriramSubramanian no the pitch change doesn't induce a climb or descent just from the orientation of the thrust line.  I added some text to my post.  The A/P pretty much does what a human would do to hold an altitude, but is a bit less fussy about working the trim and will tolerate holding a bit of force continuously, but otherwise it's the same control inputs.

Awesome! That really sums it up neatly. Thanks a lot!

That's easy.  Just push the nose down a teensy bit.  (or rather, pull the nose up less.) That will reduce AoA, reduce lift slightly, prevent you from climbing and will hold altitude.
"But then, we'll speed up!" Correct.  So, reduce thrust as needed.
Another option is to intentionally climb.  Then, you enjoy the fuel economy benefit of higher altitude.   A large plane heavy with fuel is unable to reach its max design altitude.  So, as the aircraft sheds fuel, it can seek higher and higher altitudes, to gain the fuel efficiency of pushing through thinner air.  It's common for a long-haul flight to start at 31,000 feet, then climb to 33, 35, 37, 39,000  as fuel is burned off.

As pointed out by HiddenWindshield, the pilot will pitch the nose down slightly as the fuel load is burned off. The pilot does this using the elevator trim control and will from time to time in a long flight add nose-down trim to maintain zero vertical speed. With less load, less lift is required, and trimming away the unnecessary lift will cause the plane's speed to rise slightly unless the pilot reduces power.

There are a lot of things that affect lift beyond just the speed and air density. For instance, the angle of attack. As the weight of the aircraft decreases, the pilot (or autopilot) will pitch the nose down slightly to reduce the AoA, and therefore lift.

As pointed out in other responses, there are many factors that influence how and why an aircraft will need constant adjustments made to maintain speed and altitude while cruising.
You are correct in assuming that as the mass decreases as you burn off fuel, there will be less lift required to counteract gravity. It will also take less thrust to counteract the drag of the air, so either the pilot or the autopilot will need to make continuous adjustments inflight, small though they may be.
One thing that has not yet been mentioned is Air Traffic Control. Each aircraft at "cruise altitude" (which is usually above 18,000 feet, or Flight Level 180, and most often above 29,000 feet, FL290) is in controlled airspace, and has an altitude assigned to them by Air Traffic Control (ATC). You must maintain this altitude to maintain proper separation from other aircraft. Of lesser importance, but still something to keep in mind, is that ATC expects you to maintain your airspeed while cruising, and not to significantly increase or decrease your speed unless you notify them. Again, this is to allow them to maintain separation between you and other aircraft.
With that in mind, I hope you can better understand why the constant pitch and throttle adjustments mentioned above, even if small, are essential.


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What are the factors behind crew pairing and crew rostering?
What are the factors to be taken in to consideration for crew pairing and crew rostering?

This is an airline personnel question, not aviation.

@CrossRoads, questions about airline crew have always been considered on-topic, the crew selection and pairing question should be okay.

It is very, very complicated.
It is so complicated that an entire department is dedicated to this task. Below I would provide a glimpse of crew rostering.
Basics
Most airlines provide roster on a monthly basis. The flights that will be flown are well planned beforehand. From this, a minimum number of crew must be on each flight, for example a Captain, a First Officer, and certain number of flight attendants. Many requirements and restrictions must be meet. For example:

Crew must be qualified to perform their duties (more on that below).
Duty time, rest time, ground time, turn-around time must be legal, and preferably, reasonable.
Mandatory crew may be different for different aircraft types. For example, a B777 on a long haul flight may require 2 Captains, 1 First Officer, 1 Second Officer and 10 flight attendants. A A320 on a short flight would require 1 Captain, 1 Second Officer, and 4 attendants.
Generally speaking, cabin crew can be swapped between aircraft types, such as between Airbus and Boeing. Pilots are current (legally qualified to fly commercially) only on a specific type.
Crew are sometimes assigned to ad-hoc duties, such as accepting delivery of a new aircraft.
Besides flying, crew would also have training duties, line-check duties and simulator duties.

Constraints
Mostly for safety reasons (and also for risk reduction), some pairing combinations are not allowed either legally or by company requirement. For example:

A pilot aged 60 or above must be paired with another pilot below the age of 60.
A newly joined Captain (defined as having flown X sectors or less) must not be paired with a newly joined First Officer or Second Officer.
Similarly, the percentage of newly joined cabin crew (defined as having flown X sectors or less) should not exceed a certain number.
Some flights may call for an experienced crew, for example a new route (no one has landed at that airport before) or charter flight.

Qualifications
There are various qualification requirements that must be fulfilled for a crew to legally perform duty on commercial flight. For example,

Captain must have made a takeoff and landing within the previous 30 days.
First Officer or Second Officer must have made 3 takeoffs and landings within the previous 90 days.
Certain qualifications (e.g. takeoff and landing requirements above) expire, but can be renewed on a commercial flight. You'd want to plan it so that the pilot has a chance to attempt it during duty hours, instead of going through additional training after it has expired.
Certain qualifications must be renewed through training, for example in a simulator. You'd also want to plan it so it is renewed before flying status is invalidated. Remember that the trainer may also have flying duties as well.
Route may require additional ratings. For example crew must be ILS CAT-III trained to use it. If it flies over Chinese airspace crew must be trained to fly metric system. If it flies over the Atlantic crew must be trained to fly North Atlantic Tracks.
Line-check needs to be performed every X number of days.
Crew must possess the necessary nationality or Visa for the departure / destination airport.

Ad-hoc / stand-by
Unforeseen circumstances such as mechanical problems, adverse weather, on-route diversions, crew sickness etc. may require last minute changes to the roster list. This is undesirable, so a certain number of crew would be on "stand-by" duty, meaning there is no duty assignment on that particular day, but they must be available at a particular airport within X minutes shall the need arises. There are legal requirements on stand-by time and duty time after stand-by as well.
If the department does not plan well, there may be insufficient number of crew members (i.e. not enough manpower) to serve all flights.
Improvements
Fatigue is becoming more of a hot topic in aviation. Just because a duty assignment is legal and within company policy, does not necessary mean it is good. Software can be used to analyze factors such as timezone shifts, jet lags, day night patterns etc. to identify and avoid possible fatigue scenarios.
On certain routes, it may be beneficial to assign crew who can speak certain foreign languages. For example, if statistics show that a portion of the passengers on a route are Koreans, it would be better to assign it to cabin crew who can also speak Korean. It may even be a company requirement that the cabin crew together must speak a few chosen languages, so at least there is someone who can translate.
How it is done
As you can imagine, given this many constraints, the roster is done by software. It is mainly a constraint satisfying problem, which can be solved by many well-established algorithms in computer science.
Some rules are "hard rules", meaning they are legal requirements. Some are "soft rules", they are best accommodated to provide optimal performance but may be violated when necessary. For example it is preferable to fairly allocate flight and duty time. It is also preferable to fulfill any route preference requested, but that may not be possible given other constraints.